1) Field of the Invention
The present invention relates to an analog/digital converter, and particularly relates to a delta sigma modulation type analog/digital converter which is preferably used in the field of PCM (Pulse-Code Modulation) audio system.
2) Prior Art
In FIGS. 1 to 3, are shown the constructions of general type of delta-sigma modulation A/D converters. The A/D converter shown in FIG. 1 comprises a subtraction portion 1, an integrating portion 2, a signal digitalizing portion 3, and a feedback analog signal generating portion 4. In this A/D converter, input analog signals are converted into digital signals by conducting a first order delta-sigma modulation. In such a construction, a process for changing the integral calculus obtained in the integrating portion 2 into one bit digital signals in the signal digitalizing portion 3, and a process for generating a feedback analog signal, which is equivalent to the outputted digital signal, in the feedback analog signal generating portion 4, and a process for integrating a signal obtained by subtracting the feedback analog signal from the inputted analog signal in the integrating portion 2 are successively conducted at every predetermined sampling period. As a result of repeating these processes, one bit digital signals can be obtained which are supplied to a pulse-number modulator with the aid of the input analog signal.
FIGS. 2 and 3 show other constructions of the general type A/D converters where the order delta-sigma modulation is conducted. These A/D converters shown in FIGS. 2 and 3 also comprise a signal subtracting portion(s) 1, an integrating portion 2, a signal digitalizing portion 3, and a feedback analog signal generating portion 4. In FIG. 2, the numerical reference 5 represents an adder. As shown in FIGS. 2 and 3, in the case that the third order delta-sigma modulation is conducted, three integrating portions 2 are necessary.
In audio equipment, it is frequently required to convert analog signals for a plurality of channels into digital signals. In such a case, the corresponding number of the A/D converters to the number of the channels should be provided in a parallel manner, so that the A/D converting processes can be conducted in each of the A/D converters for each channel.
In such a case, the circuits for the signal processing system of the audio equipment become large and the price of the equipment becomes expensive. Additionally, when the circuits for the A/D converting processes for a plurality of channels are constituted of an LSI (Large Integrated Circuit), the area of the semiconductor chip of the LSI becomes large, so that the price for the LSI per se becomes expensive.
Further, in order to constitute an A/D converter where a highly-ordered delta-sigma modulation can be conducted, the same number of integrators as the number of the orders are necessary. Therefore, the size of the circuit of the A/D converter for the highly-ordered delta-sigma modulation becomes large and the price of the A/D converter becomes expensive.
The present invention has as its purpose to provide an A/D converter where analog signals for a plurality of channels can be converted into digital signals without making the size of the circuit large.
Another purpose of the present invention is to provide an A/D converter where analog signals can be converted into digital signals by highly-ordered integrating processes without making the size of the circuit thereof large.